Sulfur and phosphorous-containing compounds are commonly used to prevent wear and improve the load-carrying capacities of lubricants. However, sulfur and phosphorous-containing compounds have the drawback in that they can be corrosive, which impairs the performance of lubricants into which they are incorporated.
Corrosion also is a greater concern in outdoor applications of such lubricants involving wet, moist or humid environments. For instance, wind turbine applications, such as those used in wind farms or wind plants as an alternative renewable source of energy, are increasingly attracting more interest. Wind-electric turbine generators, also known as wind turbines, use the energy contained in the wind to spin a rotor (i.e., blades and hub). As the air flows past the rotor of a wind turbine, the rotor spins and drives the shaft of an electric generator to produce electricity. Wind turbine usage is increasing throughout the world, with about a three-fold increase in power generated from wind turbines occurring between 1998 and 2001 alone. Pohlen, J., “Lubricants for Wind Power Plants”, NLGI Spokesman 67(2), 8-16, (2003). To create this energy using a conventional wind turbine, a gear-box is typically placed between the rotor of the wind turbine and the rotor of a generator. More specifically, the gear-box connects a low-speed shaft turned by the wind turbine rotor at about 30 to 60 rotations per minute to a high speed shaft that drives the generator to increase the rotational speed up to about 1200 to 1600 rpm, the rotational speed required by most generators to produce electricity. This geared solution can result in a torque through the system of close to 2 million N*m. Pohlen, J., “Lubricants for Wind Power Plants”, NLGI Spokesman 67(2), 8-16, (2003). This high torque can put a large amount of stress on the gears and bearings in the geared wind turbine. Wind turbine oils are desired that will enhance the fatigue life of both the bearings and gears in the wind turbines.
In addition, wind turbines normally are located where wind is most plentiful, including coastlines and offshore locations, as well as in inland locations that are occasioned by atmospheric moisture in the form of rain and/or humidity. These severe environments may place additional performance demands on the lubricants required for wind turbines. For instance, exposure of such wind turbine devices to the elements as part of their basic functionality increases the risk of corrosion problems in the mechanical parts of the wind turbine. Furthermore, due to the remote locations of many wind turbines, frequent replacement of wind turbine oils is not practical or cost-effective, and thus these oils need to be more oxidatively stable than industrial lubricants used in many other applications. In addition, there is a high probability that water may contaminate the lubricating fluid. Ideally wind turbine lubricating oils should prevent corrosion, be hydrolytically stable, and increase the fatigue life of gears and bearings in the presence of water. Due to these concerns, wind turbine manufacturers are developing new lubricant specifications for wind turbine oils imposing very stringent fatigue life requirements, and also requirements for performance testing in the presence of water.
Gearless direct drive wind turbines have been developed, which have the advantage of having less moving parts to maintain, but have their own drawbacks of generally being heavier and generally being open models allowing cold air to pass through, which may pose an increased risk of corrosion, especially in offshore installations. In any event, it is expected that both types of wind turbines will co-exist for some time. Therefore, wind turbine oils that would enhance the fatigue life of bearings and gears in gear-boxes used in geared wind turbines would increase the opportunities to use the geared solution in the most efficient, reliable and cost-effective manner.
More generally, inasmuch as gear oils are often subjected to prolonged periods of use between any maintenance and service intervals, such as in wind turbines, as well as in vehicular differentials and like devices, it generally is important to provide gear oil additive systems capable of rendering improved service performance over lengthy durations of time. Moreover, additive combinations that improve the anti-corrosion properties of lubricating fluids containing sulfur and/or phosphorous compounds over long periods of time in service are needed, especially for outdoor applications, such as in geared wind turbines. In addition, while acceptable performance of the lubricating oil is needed, it is also highly desirable that the additive or additives be cost-attractive and conveniently manufactured.